Frequency mixers combine input signals to produce an output signal. Mixers are frequently used to down-convert a radio frequency (RF) signal to a lower intermediate frequency (IF) signal or up-convert an IF signal to a higher RF signal. Mixers down-convert by mixing an RF signal with a local oscillator (LO) signal. Mixers up-convert by mixing an IF signal with an LO signal.
Mixers can be either fundamental or subharmonic. A fundamental mixer down-converts an RF signal to an IF signal by mixing the RF signal with the fundamental frequency of an LO signal. A fundamental mixer up-converts an IF signal to an RF signal by mixing an IF signal with the fundamental frequency of the LO signal. The fundamental mixer produces a mixed signal that is rich in harmonics. If the fundamental mixer is a down-converter, the mixed signal will have harmonics that are the sums and differences of the harmonics of the RF and LO signals (i.e. nflo±mfrf). If the fundamental mixer is an up-converter, the mixed signal will have harmonics that are the sums and differences of the harmonics of the IF and LO signals (i.e. nflo±mfif). Generally, the mixed signal is then filtered to produce the RF signal or the IF signal.
A subharmonic mixer operates similar to a fundamental mixer except that the subharmonic mixer mixes an RF signal or an IF signal with a harmonic of the fundamental frequency of the LO signal, usually the 2nd harmonic. A subharmonic mixer down-converts an RF signal to an IF signal by mixing the RF signal with a harmonic of the fundamental frequency of the LO signal. This produces a mixed signal having harmonics that are the sum and difference of the harmonics of the RF and LO signals (i.e. nflo±mfrf). A subharmonic mixer up-converts an IF signal to an RF signal by mixing an IF signal with a harmonic of the fundamental frequency of the LO signal. This produces a mixed signal having harmonics that are the sum and difference of the harmonics of the IF and LO signals (i.e. nflo±mfif). The mixed signal is then filtered to produce the IF signal or the RF signal.
Fundamental mixers are impractical for many very high frequency applications. A fundamental mixer for a high frequency application may require LO signals with a very high fundamental frequency. LO's that produce signals with very high fundamental frequencies are often prohibitively expensive or unavailable. Instead, a subharmonic mixer is frequently used. Subharmonic mixers exploit the harmonics of the LO signal and thus can use LO signals (and thus LO's) having a fundamental frequency much lower than the LO of an equivalent fundamental mixer. This makes the subharmonic mixer a more practical choice than a fundamental mixer for many high frequency applications. Nevertheless, one skilled in the art will recognize that there are trade off's associated with the selection of either a fundamental or subharmonic mixer, and will exercise judgment in determining which type of mixer is most appropriate for a particular application.
Although several types of subharmonic mixers exist, one type of subharmonic mixer features an anti-parallel diode pair. In an exemplary down-converter (up-converter) mixer the RF (IF) signal may be fed into one end of the anti-parallel diode pair and the LO signal into the other end of the anti-parallel diode pair. The anti-parallel diode pair forms a current conduction loop. The conduction loop is tapped to extract a mixed signal. Typically, the LO signal dominates, therefore generally when the LO voltage is positive, current is conducted through the first diode in the diode pair. When the LO voltage is negative, current is conducted through the second diode in the diode pair.
The current-voltage (I-V) conduction characteristic of each of the diodes is nonlinear—producing currents rich in harmonics. Ideally, the I-V characteristics of the diodes are identical, the even harmonics of the current circulate within the diode loop, and the tapped mixed signal only has the odd harmonics present. The mixed signal may then be filtered to generate an IF signal.
In practice, however, the I-V characteristics of the first and second diodes in the anti-parallel diode pair are not exactly the same. The first and second diodes may for example have different saturation currents (Is) or a different slope parameter (α) due to less than exact manufacturing tolerances. Also, manufacturing tolerances may also cause the junction capacitance (Cj) to be different from diode to diode. This asymmetry in diode conduction characteristics and junction capacitance may result in leakage of even harmonics into the mixed signal.
Even harmonic frequency spurs in the mixed signal may be especially problematic since many down-converting (up-converting) subharmonic mixers feature an LO having a frequency of about one half the RF (IF) frequency. This produces a mixed signal having second harmonic (2LO) spur close to the RF (IF) frequency. This 2LO frequency spur is especially unwanted since it may interfere with other RF (IF) signals.
Thus, there is a need for a subharmonic mixer that is configured to suppress even harmonic frequency spurs, particularly second harmonic spurs. Stated another way, there is a need for subharmonic mixers that reduce the leakage of even harmonics into the mixed signal. Similarly, there is a need for reducing the leakage of even harmonics into the mixed signal of fundamental mixers.